Envelope tracking power amplifier systems are known in the art, and generally comprise the provision of an input signal to be amplified on an RF input path to a signal input of a power amplifier, and an envelope path for generating a modulated power supply based on the envelope of the input signal, with the modulated power supply being provided to a power supply input of the power amplifier.
In applications envelope tracking power amplifier systems are desired to be highly efficient. In order to be highly efficient, the amplifier component of such systems is required to be highly efficient. In particular efficient, low noise, high bandwidth amplifiers are required by envelope tracking systems.
A current conveyor acts as a conduit for current from one node to another, converting one voltage level to another voltage level, and acting as a buffer. The current conveyor buffers a given current between two nodes at a different voltage.
Current conveyors are used in high frequency applications where a conventional operational amplifier cannot be used because the conventional design is limited by gain-bandwidth products.
Current conveyors are an important component of high-efficiency amplifiers. A current conveyor is required in an amplifier to provide high gain across very high bandwidths, which may be needed to minimise the impact of the noise and spurious signals generated by switch mode circuits used elsewhere in the envelope tracking modulator.
FIG. 1 illustrates an ideal current conveyor. As illustrated in FIG. 1, an input signal is provided on a line 102 to one terminal of a voltage source 104, the other terminal of which is connected to electrical ground. A current source 106 has one terminal connected to electrical ground, and the other terminal generates a signal on line 108. The signal on line 102 is the input signal, and the signal on line 108 is the output signal. Current is detected in voltage source 104 and replicated in current source 106, so that the voltage at the output can be different to the voltage at the input.
A current conveyor circuit should ideally have a zero input impedance to prevent loss in the input circuit. A current conveyor should also ideally have an infinite output impedance to prevent loss in the output circuit.
Whilst FIG. 1 illustrates an ideal implementation of a current conveyor, with reference to FIG. 2 there is illustrated a typical implementation of a current conveyor.
As is illustrated in FIG. 2, a CMOS transistor 202 has a source terminal connected to electrical ground, and a drain terminal connected to one terminal of a current source 208. A CMOS transistor 204 has a source connected to one terminal of a constant current source 206, the other terminal of the current source being connected to electrical ground. The drain terminal of the CMOS transistor 204 is connected to one terminal of a current source 210 which provides a high impedance. The other terminals of the current sources 208 and 210 are connected to a supply voltage VS. The gate of the CMOS transistor 204 is connected to the drain of the transistor 202. The gate of the CMOS transistor 202 is connected to an input signal on line 212. The input signal on line 212 is also connected to the source of the CMOS transistor 204. An output signal is provided on a line 214 which is connected to the drain of the CMOS transistor 204.
A typical current conveyor as illustrated in FIG. 2 has unity gain.
The typical current conveyor as illustrated in FIG. 2 uses a CMOS common gate transistor 204 with a regulated gate voltage provided by CMOS transistor 202 to achieve a low input impedance and a high output impedance.
As transistor 202 switches on, the voltage at the gate of transistor 204 decreases and the voltage at the source of transistor 204 also decreases, counteracting the input voltage increase, and lowering the input impedance. The input approximates a voltage source as indicated in FIG. 1.
With the current conveyor arrangement of FIG. 2, the optimal performance is achieved when the input current and the output current of the current conveyor are equal and thus the current gain is unity. An input impedance of greater than zero, or an output impedance of less than infinity, will reduce the current conveyor's gain to less than unity.
In an amplifier where high gain across a wide bandwidth is advantageous, the usefulness of the unity gain current conveyor is limited. The unity gain provided by such typical current conveyor arrangements is a significant disadvantage in amplifier designs where high gain across a wide bandwidth is required. In existing systems, in order to provide high gain, the current conveyor's limited gain has to be compensated for by increasing the gain requirements of the amplifier sections that precede and follow the current conveyor, in order to achieve a required amplifier gain. These amplifier sections then require an excessive current to provide this gain across a wide bandwidth, and increasing complexity and die area, and reducing efficiency.
It is an aim of the present invention to provide an improved current conveyor for an amplifier.